A thin steel pipe has heretofore been used as a fuel pipe for cars. As a pipe for lower gasoline, sour gasoline, alcohol and alcohol-mixed fuel, a welded pipe is used, which is manufactured by electrically nickel-plating a steel strip on at least one surface thereof for the purpose of improving the rust resistance of the inner surface of the pipe, shaping the thus nickel-plated steel strip into a pipe by roll forming with the nickel-plated surface being inside of the pipe, welding the butt-jointed part by electric resistance butt welding with electrode rolls or the like to form a welded pipe, and annealing it.
However, since the welded pipe having a nickel-plated layer on the inner surface thereof is manufactured by welding the butt-jointed part, after shaping into a pipe form, by electric resistance welding or the like, the bead part to be formed on the inner surface of the pipe causes discontinuity of the nickel-plated layer so that the steel foundation is locally exposed out (FIG. 3) or pin holes as existing in the nickel-plated layer remain as they are. Therefore, the part has a problem of being easily corroded by water as being in alcohol or alcohol-mixed fuel or by organic acids to be formed by oxidation of gasoline or by decomposition of alcohol.
The present invention is to overcome the above-mentioned problem, and the object of the present invention is to provide a welded pipe, which has an excellent corrosion resistance since even the bead part is coated with a plated metal layer and pin poles in the plated metal layer are also coated and which has an excellent workability for formation of terminals, and also to provide a method of manufacturing such a welded pipe.
The present inventor repeatedly studied for the purpose of overcoming the above-mentioned problems and of attaining the above-mentioned object and, as a result, has found that the object can be attained by forming a welded pipe from a steel strip as coated twice with a first plate layer of one of Ni, Co and Ni- or Co-base alloys and then with a second plate layer of a metal or alloy having a lower melting point than the metals or alloys of forming the first plate layer, or one selected from Sn, Sn-Zn, Ni-P and other low melting point metals and alloys. On the basis of the finding, he has attained the present invention.
Specifically, in accordance with the first embodiment of the present invention, there is provided a welded pipe having an excellent inner surface corrosion resistance, the inner surface of which at least except the bead part has a first plate layer of one of Ni, Co and Ni- or Co-base alloys and the inner surface of which including the bead part additionally has, on the first plate layer, a second plate layer of a metal or alloy having a lower melting point than the metal or alloy of forming the first plate layer. As the second embodiment of the present invention, there is provided a method of manufacturing a welded pipe having an excellent inner surface corrosion resistance, in which a first plate layer of one of Ni, Co and Ni- or Co base alloys is formed on at least one surface of a steel strip, then a second plate layer of a metal or alloy having a lower melting point than the metal or alloy of forming the first plate layer is formed on the first plate layer, and thereafter the thus plated steel strip is formed into a pipe with the plated surface being inside and then heat-treated.